Polyvinyl alcohol (hereinafter, may be abbreviated as “PVA”) is known as a water-soluble synthetic polymer. PVA is particularly excellent in strength and film forming properties compared with other synthetic polymers. PVA is therefore used as a material for film and fiber, an additive for paper and fiber processing, an adhesive, a stabilizer for emulsion polymerization and suspension polymerization, a binder for inorganics, and the like. PVA is thus heavily used in various applications.
In recent years, a method of using chemicals, such as household chemicals, like detergents, bleaches, and toiletries, agrochemicals, and industrial chemicals, by seal-packaging (unit packaging) in a water-soluble film in each unit amount has become popular. The method is described below. In advance, the content is seal-packaged (unit packaged) in a water-soluble film in each certain amount. For use, it is thrown into water remaining in the form of package. By dissolution of the water-soluble film, the content is dissolved or dispersed in water. The unit package is advantageous in that it allows use without directly touching hazardous chemicals in use, no measurement is required in use because the content is packaged in each certain amount, the container for packaging the chemical does not have to be disposed after use, and the like.
Since PVA films generally have advantages of being tough, excellent in transparency, and good in printability, they are often conventionally used as water-soluble films for unit package. In PVA, however, with a higher degree of saponification, the crystallinity increases and the ratio of crystal parts that are not dissolved in cold water increases. Therefore, for applications requiring cold water solubility, not PVA having a high degree of saponification, called a completely saponified type, but unmodified partially saponified PVA has been used. Water-soluble films using the unmodified partially saponified PVA have advantages of being readily dissolved in cold water and warm water, excellent in mechanical strength, and the like.
In recent years, from the perspective of workability, chemical resistance, environmental protection, and the like, there is a demand for a water-soluble film that simultaneously satisfies performance, such as having a faster rate of dissolution in cold water, being less easily broken by impact, having water solubility varied less with time during storage, and being good in biodegradability. Conventional unmodified partially saponified PVA films, however, have a problem that the cold water solubility decreases during storage for a long period. The water solubility is considered to decrease because the crystal gradually grows during storage. Moreover, when an alkaline or acidic substance is packaged in such unmodified partially saponified PVA film, saponification of acetate groups remaining in the partially saponified PVA occurs during the storage and crystallization proceeds to insolubilize the film. The required performance has thus not been obtained. In addition, when a chlorine containing compound, such as an agrochemical and an antiseptic, is packaged in a film using the unmodified partially saponified PVA to be left for a long period, the film is colored and cured and also the water solubility decreases with time to be insoluble or poorly soluble in water. The content is therefore not dissolved or dispersed in water in the state of remaining packaged in the film, and thus the original purpose is not achieved.
As a solution to such problems, a water-soluble film of modified PVA having an anionic group, such as a carboxyl group, is proposed. For example, Patent Document 1 discloses a water-soluble film of modified PVA having a monomer unit containing a carboxyl group, such as itaconic acid and maleic acid. Patent Document 2 discloses a water-soluble film of a composition containing modified PVA, having a monomer unit containing a carboxyl group and/or a sulfonic acid group, and polyhydric alcohol. Further, Patent Document 3 discloses water-soluble modified PVA with a monomer unit containing acrylic acid randomly introduced therein and a package using a film of the modified PVA.
Such water-soluble film of modified PVA having an anionic group, such as a carboxyl group, however, has a disadvantage of losing cold water solubility when making contact with an acidic substance. Further, the modified PVA is sometimes gelatinized by heat. In addition, the modified PVA is capable of increasing dissolubility in cold water by increasing the carboxyl group content while a water-soluble film thereof sometimes has reduced mechanical strength, so that it used not to be preferred. There is, accordingly, a demand for modified PVA that is excellent in dispersibility in water without gelation and the like and is further capable of obtaining a film excellent in cold water solubility and mechanical strength.